


Universal Constant

by 1863



Category: Batman v Superman: Dawn of Justice, DC Extended Universe, Justice League (2017)
Genre: Accidental Travel to an Alternate Universe With an Unexpected Spouse, Developing Relationship, Extra Treat, M/M, Post-Canon
Language: English
Status: Completed
Published: 2019-08-06
Updated: 2019-08-06
Packaged: 2020-08-10 11:10:22
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,653
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/20134489
Author URL: https://archiveofourown.org/users/1863/pseuds/1863
Summary: An unknown device transports Bruce to a familiar place, where he meets an equally familiar person. But while they may seem identical on the surface, there's one very, very big difference.





	Universal Constant

**Author's Note:**

  * For [Rhovanel](https://archiveofourown.org/users/Rhovanel/gifts).

> Didn't quite make it before the collection opened, but I hope you enjoy this late treat!

"Don't even think about it, Bruce."

"Too late."

Clark's jaw tightens. "Damnit, don't be stupid. There has to be another way —"

"Yeah," Bruce agrees. "There is." He licks his lips and edges closer to the device on the workbench, pushed into a space they’d cleared in the cave. It's emitting a noise that sounds a lot like a warning and coloured lights and buttons are flashing, in an urgent kind of way, faster and faster like it’s on a countdown. "You die, or I die. And I'm pretty sure it's my turn now."

Clark inhales sharply when he realises that Bruce isn't joking.

"Be logical about this," Clark starts, but Bruce can already see his shoulders tensing, muscles contracting, right foot turning a fraction of a degree outward — all signs that Clark is getting ready to use his superspeed to beat Bruce to the punch. He makes a mental note to create a training regimen that will smooth those tells away — if they're both still alive after this, at any rate, since neither of them actually knows what the device even does. And then Bruce brings out the big guns, the one thing he knows for sure will give him the upper hand. 

He smiles. And Clark freezes, just as Bruce had known he would.

"Bruce?" he asks, suddenly uncertain. This is the reason Bruce only deploys this particular weapon when he has no other choice — it never fails to throw people off-balance, no matter who they are. Even Superman.

He doesn't waste any more time with a reply. Clark only needs a microsecond to make the whole situation go belly-up, and if there's anything that Bruce enjoys even less than Clark's penchant for self-sacrifice, it's that same penchant spoiling one of his plans.

Bruce dives right on top of the device. The last thing he sees before the world winks out of existence is the unexpected look of regret — and entirely expected lack of surprise — on Clark's stricken, wide-eyed face.

***

There's no period of unconsciousness, no moment of disorientation — one second he's in the cave and the next, he's —

_Still_ in the cave.

Bruce slowly straightens up, every sense on high alert despite the familiarity of his surroundings. It's definitely the cave — the workstations, the spare suits, the weapons and gadgets, even the smell — of metal and stone and the faintly stale quality of recycled air — all of that is here, all of that is the same. But it doesn't take Bruce long to realise that although he's in the cave, he's not in _his _cave.

The differences become more and more obvious the more he looks around. It's brighter, warmer — not so much in terms of temperature or light, but in atmosphere. It's not quite as clinical here, not quite as utilitarian; still definitely a place of work but less sterile, somehow.

The clearest sign of all, though, the one that makes it really hit home that he’s somewhere he absolutely does not belong, is the Robin suit. It's not in the same place, encased in an alcove built right into the wall instead of displayed in front of the staircase. In this cave, he wouldn’t be forced to walk right past it every time he comes and goes. 

Bruce swallows hard when he sees it. Because it's not just the position of the display that's changed, it's also the fact that the suit is clean. No more splashes of blood on the torn cape, no more graffiti scrawled across the chest. No crowbar or playing card on the floor next to it, either — just the suit alone, bathed in warm gold light, as pristine as the day he made it.

"You had one, too?"

Bruce whirls around. Someone steps forward, out of the shadows that pool in a hidden corner where he’d always wanted to install another entrance. And _now_ the disorientation comes, now he wonders if he actually did pass out when he dove for the device and ended up in some kind of hallucination or fever dream instead.

"You're not dreaming, Bruce." The other man gives him a quick once-over, taking in the batsuit he’s wearing but the lack of a cowl. "You don't seem any younger than me," he muses. "Or any older, so it's probably not time travel. I'm guessing…” He trails off and looks thoughtful. “Alternate universe, maybe?" 

There are any number of counterarguments that Bruce could throw back at him — things backed up by hard science and immutable, provable fact — but he knows, deep down, that it would be futile. His gut is telling him that this man is right, that this is exactly what it appears to be, and illogical or not, Bruce has learned to trust his gut even when the evidence might say otherwise.

He sweeps his gaze over the man too, a thorough look up and down. Hair a little longer, maybe, but as far as Bruce can tell it’s the only physical difference between them. Same build, same face, even the same clothes — an ordinary suit, not a batsuit, but still a suit that Bruce recognises as one of his own.

"So," Bruce says. "You're me. From a different universe?"

"Evidently." Alt-Bruce looks faintly amused. "Is this your first time? Meeting another version of yourself, I mean."

"This has happened to you before?"

"Come on, Bruce," his alternate replies. "Are you really that surprised? We lead… unusual lives."

Bruce shakes his head. "This is insane," he mutters, but can't deny that Alt-Bruce has a point. He counts an alien and superpowered royalty amongst his teammates, not to mention people who can move in ways that defy the laws of physics and command technology like it's an extension of their own mind. The existence of parallel universes isn’t really that much of a leap.

Still, he wouldn't be Batman if he didn't double check, if he didn't make absolutely, one hundred percent sure.

He glances back at the Robin suit, at the display embedded in the wall.

"Is he—"

"Dead,” Alt-Bruce says. “The Joker killed him." He tilts his head a little, sizing Bruce up. "Go ahead and ask, Bruce. It's the only way you'll know for sure."

"Where is he buried?"

Aside from Bruce himself, only Alfred knows the answer to that question.

"Next to our parents," is the immediate reply, and really, Bruce isn't even that surprised. "He was family, after all."

Bruce has to take a breath at that but doesn't otherwise respond, pushing the grief and the guilt aside. They're old wounds; he can tear them open any time. Right now, there are more pressing matters at hand. But there's still one other thing he wants to know, a point of difference between them he needs to understand.

"It's in a different place in my cave," he says. "His uniform, I mean. And it's not — it still looks the way it did before."

_When he died_, Bruce doesn't say, but then, he doesn't have to.

"So did mine," Alt-Bruce replies. His voice goes quiet. "I used to keep it at the foot of the stairs, so I'd have to walk past it every day. I didn't want to give myself a chance to—”

"Forget," Bruce finishes for him. "Forget that what happened was our fault." Bruce searches his eyes, trying to find where the similarities end and the differences begin. He can't even fathom touching the suit again, let alone cleaning and repairing it. "But you moved it," he adds. "You made it look like new again."

Alt-Bruce shakes his head. "Not new," he says. "Just back to the way it was, the way he wore it when he was doing what he did best. The way it's supposed to look." He lapses into silence, face clouding over with thoughts that Bruce knows only too well.

"Why did you move it?" he asks. "Why did you clean it up?"

And then, shockingly, his alternate smiles. A real smile, not the kind Bruce uses as a weapon or a deflection or a mask, but something much, much more rare. His eyes fill with warmth, a warmth that's echoed in his voice when he answers Bruce's question.

"He convinced me to," Alt-Bruce says. "Didn't outright ask, of course — you know he'd never do that. After all," he adds, smile turning a little rueful, "he knows we're not exactly good at letting go of the past. But he reminded me that there are better ways to honour someone than defining them by the way they died."

"Your Alfred sounds a little different to mine," Bruce remarks. "Mine most definitely _would_ ask, if he thought I’d actually listen for once."

Alt-Bruce frowns. "Alfred?" he repeats. "I didn’t say anything about — oh." He smiles again, but the look in his eyes has an edge of something other than amusement in it now, something more like… compassion, maybe, or sympathy. Slowly, he raises his hand — his left hand. He brings it up to the side of his face and now, no longer hidden in the shadows by his side, Bruce sees the flash of gold around his ring finger, glinting a little in the artificial light.

"You," he says blankly, staring. "You're—?" He can't even say the word, the very idea so utterly absurd.

Alt-Bruce lowers his hand again. "For a year and a half now, give or take."

A year and a half. For the past 18 months, he's been — there's been a version of himself that's been —

Bruce just stares. Alternate dimensions were one thing, but this? This he can't believe. The prospect of trusting someone enough to tell them about Batman is already so beyond the realm of possibility that the idea of even more — of wanting a future with someone, of sharing a lifetime with someone… No. That's just too—

"Who?" Bruce blinks; he hadn't intended to ask. Some part of him doesn't even want an answer.

Alt-Bruce narrows his eyes. It seems like a long time before he replies.

"Something tells me," he says slowly, "that you already know the answer to that, Bruce."

And Bruce is shaking his head in denial even before Alt-Bruce finishes speaking, even as the flutter in the pit of his stomach tells him he's wrong and that his alternate is right. He does know, of course he knows. Bruce looks away, around the cave again, anywhere but at the look of deep, deep understanding in Alt-Bruce's eyes. And now he can see it, now he sees it everywhere: all the tiny clues and pieces of evidence that he somehow missed, the first time around.

The small stack of paperbacks and dog-eared notebooks on one of the benches, neatly pushed to the side so they’re not in the way. A jacket that looks familiar but he knows isn't his, draped over the back of a workstation chair. And a pair of glasses with thick black frames, folded up and sitting on the shelf closest to the main computer, the one he uses the most. The computer that has two chairs in front of the monitor here, instead of just one.

“How,” he starts, but has to stop. Words seem beyond him right now, his tongue too big and too clumsy in his mouth, his throat too dry to make a sound. Alt-Bruce says nothing for a long time, just watching him and letting it sink in.

"He's just as afraid as you are, you know," Alt-Bruce says eventually. "But it's like missions with the League." Something brightens his eyes a little, something that makes Bruce almost want to turn away. "When one of you dives headfirst into danger, the other one always follows. Always.”

Bruce takes a deep, deep breath. It's a different universe, he reminds himself, an entirely separate, standalone reality. What happened here doesn't necessarily mean — there's no way his own life could ever —

"You're thinking it's impossible." Alt-Bruce sounds amused again, but there's something else there too, layered beneath his faint smile — a tangled sort of sadness that Bruce doesn't quite understand; a kind of quiet regret. "I did, too. But you've spent a lifetime redefining the impossible, Bruce. And frankly, so has he."

"You don't understand," Bruce insists, and isn't sure if he's trying to convince his alternate or himself. "We're — he's not even —" He stops and takes another deep breath, willing the words to come together and then forcing himself to speak them out loud. "You're right. On a mission, I'd follow him anywhere. But this?" He gestures to the ring on Alt-Bruce's hand. "This is different. I'm not you. I can't follow him there, even if he does dive first."

But Alt-Bruce's smile only widens, the flicker of sadness falling away. 

"I thought that, too," he says. "But that's the thing about Clark, isn’t it? We always underestimate just how stubborn he can be."

"Are you saying he didn't take no for an answer?" Clark might be stubborn, yes, but he was also sharply observant, much more so than most people gave him credit for. "That proves it, then," Bruce says. "Your Clark isn't like the one in my world. If he saw that I wasn't interested, then he'd never —"

"You are, though."

Alt-Bruce looks at him steadily. There's no doubt in his voice at all and Bruce knows it's pointless to protest.

"He'll see that," Alt-Bruce continues. "And I know what you're thinking — that you'll be able to lie to him just as easily as you can lie to anyone else. But you're wrong, because he's _not_ anyone else, not to you. And I don't just mean because he isn't human."

Bruce runs a hand through his hair. The denials are still on the tip of his tongue, an endless stream of them bubbling up and threatening to spill out. But he can’t dismiss the absolute certainty in Alt-Bruce's eyes, or the rock-solid faith in his voice. Because it _is_ faith — an unshakeable belief that what he has is something that Bruce can have, too. Will have. Should.

"Did he… dive first?" Bruce asks, trying to find some sliver of that same faith within himself, something to hold on to and remember if the time ever comes. "Your Clark, I mean."

Alt-Bruce laughs quietly. "Oh, he dove," he replies. "Over and over and over again, because as stubborn as he is, he's got nothing on us. But…" He trails off and looks Bruce in the eye, the smile fading as his voice turns serious. "There’s a point where even _his_ patience will run out, when your stubbornness stops motivating him and starts hurting him instead. And constantly coming up against walls as thick as yours, Bruce, will wear even a Superman down."

"How will I know when —"

"You’ll know. Believe me, you'll —" He cuts himself off and takes a deep breath of his own. "You’ll know when it’s almost too late."

The strange regret is back in eyes again and this time, Bruce's stomach clenches when he sees it, because now he understands what it means. It's not regret for anything that he did — it's regret for things he didn't do, for being the cause of someone's suffering until it was almost too much for them to bear.

"But listen," Alt-Bruce adds, shaking his head a little. "You're right. I don’t know if things are exactly the same in your universe as they are in mine. Maybe you’re different. Maybe he’s different. But given what we — me and my Clark, I mean — have gone through, what we had to admit to ourselves, and to each other, to get to where we are now..." He pauses and touches the ring, twisting it around his finger in a way that almost seems familiar, like an unconscious habit that Bruce didn’t know he had. “The truest thing I can tell you," he says, "is that if there are such things as constants in every universe, it wouldn’t surprise me if this was one of them.”

Bruce stares at him. "Are you saying —"

"You know what I’m saying," Alt-Bruce interrupts. "You know _exactly_ what I’m saying."

Bruce looks down at the device, still on the workbench and still blinking with dozens of tiny coloured lights. "You weren't surprised when I turned up here," he says slowly. "You suspected I was from an alternate universe from the start."

"Clark might be the one who dove before I did," Alt-Bruce replies, "but we both needed a little push first. And you know as well as I do that no one pushes us more than we do ourselves."

Bruce shakes his head again. "This is insane," he mutters, for a second time.

Alt-Bruce laughs suddenly, a wide grin breaking out over his face. And for a moment Bruce can only stare, frozen by the sight and struck by the sound. Somewhere along the line, he'd forgotten that he could look like that. 

"Yeah," Alt-Bruce agrees, still laughing a little. "It is. And I can honestly say that I wouldn’t have it any other way."

A high-pitched whine cuts through the air, saving Bruce from having to think of a response. He glances at the device again, the lights flashing more furiously now as the whine gets louder and louder.

"Sounds like your ride home is here."

Bruce looks up, into eyes that are almost exactly the same as his own. Just a little brighter, maybe, a little warmer. A little hap—

"Good luck to you, Bruce," his alternate says, and nods.

Bruce nods back. "And you."

***

A strong grip encases his arms as soon as the world comes back, a grip that Bruce knows is strong enough to crush bone. Right now, though, it’s only strong enough to make sure he won't disappear again. So much self-control, Bruce thinks. So much strength and power, deliberately held back. The thought makes it seem a little less impossible that Clark might be holding other things back, too. 

"I'm fine," he says. 

The hands at his arms fall away as he straightens up. Bruce is only just able to stop himself from glancing down at Clark’s fingers, to look for a ring that he knows won't be there. Not here. Not ye—

"Only through sheer dumb luck, probably," Clark replies, sounding as frustrated as he is relieved. "What the hell were you thinking, Bruce? You could've —"

"But I didn't." Clark goes quiet and Bruce finally looks up, into Clark's face. There's an oddly tense expression there, a worry that should be gone now that the situation is under control again. "How did you bring me back?" 

Clark shrugs and looks away, back at the device on the bench. It’s dark and silent now, no longer a danger, just another strange gadget in the cave. 

"That was sheer dumb luck, too," he admits. "I just kept pressing buttons on it until the right you came back."

Bruce frowns. "The right me?"

"Yeah." Clark seems to hesitate. "Lots of Bruces came back, but they were all the wrong ones. Different versions, from different… I'm not sure what happened, or how it worked." He meets Bruce's eyes again, but only briefly. "I started to wonder if I'd ever find the right one again."

"You did, though," Bruce says, not letting himself overthink it for once. "I appreciate it," he adds. "You not giving up on me, I mean.”

Clark goes still for a moment, then shrugs again. "You'd do the same for me. Right?" 

"Of course." It's the truth, they both know that by now, even if they’d never actually said so out loud before. But the tension in Clark's face is still there, as is the worry. "Did you… did you speak to any of them?" Bruce asks. "The other versions that came back before I did?"

There's a very long pause.

"Yes."

Bruce keeps his voice casual, aims for polite interest and nothing more. "They say anything interesting?"

And Clark looks away again, but not at the device on the workbench, or anywhere else in the cave. Instead he looks down, at Bruce's hands. At Bruce's left hand.

"Yes," Clark repeats, and lifts his head.

It's only twenty-two years of training that lets Bruce keep his pulse steady and his breathing under control and even then, it’s a struggle. He opens his mouth to say something, not entirely sure what might come out, but Clark gets there first.

"I could tell you about it, some time. If you're interested." Clark squares his shoulders, like he does when he's getting ready to charge into a dangerous situation, like when he thinks he’ll be confronted with a fight. "Over dinner, maybe?"

His back is straight and his gaze is direct, but there's a definite uncertainty bleeding in through the cracks, an apprehension he can’t quite hide. And yet, he still looks as determined as ever. Bruce remembers what his alternate said: _one of you dives and the other one follows. Always._

But that ignores the fact that only one of them can fly. 

Bruce might not be able to defy gravity like that, but he does know how to take a leap. He does it all the time — launching himself off the sides of buildings and into the darkness beyond, regardless of the danger he might find. But more importantly, he also knows how to land.

"Dinner?" Bruce repeats, and braces for impact.

Clark swallows but nods all the same, refusing, this time, to look away.

One more push, Bruce thinks, and doesn't look away either. One more leap. Taking the initial jump can be hard sometimes, but once he's in the air, it's always easier than he thought it would be. And for a few short moments, it feels just like flying.

"Sounds good," Bruce says. "It's a date.”

Clark sucks in a breath. It’s an answer, not a question, and going by the smile he gets in response, Bruce is pretty sure Clark knows that, too.


End file.
